An output voltage value or an output current value may oscillate when the state of system suddenly changed in a power conversion system. Operation may be stopped until influence of oscillation disappears in a power conversion system without control. In a typical power conversion system, feedback control is performed to obtain stable output. For example, in a power converter of digital control type, power conversion control is performed by adjusting ON-time of a control switch. A control system of a switching power supply (90) of FIG. 13(A) consists of a control circuit (91) and a drive circuit (92). The control circuit (91) inputs values of output voltage (eo) and output current (io) of the switching power supply (90). The control circuit (91) sends a timing signal for ON-OFF switching of a transistor switch (Tr) to the drive circuit (92) as shown in FIG. 13(B). The drive circuit (92) operates the transistor switch (Tr) based on the timing signal. The timing signal of the transistor switch (Tr) is as shown in FIG. 13(B).
The output voltage (eo) may greatly change during operation not only in startup. The control circuit (91) usually performs PID control, so when the output voltage (eo) converges to the desired value, overshoot or undershoot appears in the output voltage (eo).
The output voltage (eo) may change small as shown in FIG. 14(B). In this case however, when output voltage (eo) returns to the desired value, overshoot and undershoot also appears in output voltage (eo). On the other hand, output voltage (eo) may have very slow transient response when overshoot and undershoot do not appear as shown in FIG. 14(C).
In a power supply circuit used for driving a microprocessor, it is necessary that these overshoot and undershoot should be small and the output voltage (eo) must converge to the desired value within a short time.